Polyimide and polyamide-imide resins and processes for their preparation are well known in the prior art. Such resins, which are high heat resistant polymers, have particular usefulness in the aerospace industry and as magnet wire coatings. The rate of growth and use of these resins has, however, found some limitations due to the solvents required for their application. Difficulties have been encountered in coatings applications to obtain consistent, smooth coatings on certain substrates because of such solvents. In addition, it is generally known that there are relatively few solvents that will solubilize or are compatible with polyimides or polyamide-imides. It is therefore highly desirable to provide the polyimide and polyamide-imide resins in a useful molecular weight in solvents which give solutions having desirable low viscosities and relatively high polymer resin content.
Acetophenone has been found to have a very high degree of solvency for polyimides and polyamide-imides but cannot generally be employed as the sole solvent for such polymer solutions. It has been discovered that, in addition to providing an economical solvent system, acetophenone can be used in conjunction with N-methyl pyrrolidone in amounts of up to 90% by weight of the total solvent mixture down to 5 percent solids. Clear, uniform coatings on magnet wire are obtained on curing when the acetophenone-N-methyl pyrrolidone mixed solvent system is employed with polyimide and/or polyamide-imide resins.
N-methyl pyrrolidone used in the preparation of aromatic polyamide-imide resins is also a good solvent and the polyimides and polyamide-imide polymers are often sold commercially as certain percent solids solutions in N-methyl pyrrolidone (NMP). Other common solvents such as xylene, acetone or ethanol are incompatible with the resins and can be used in very small percentages in an N-methyl pyrrolidone solution of the polymer resins. Cresylic-phenol type solvents can be used but suffer from their toxic nature.
Prior art solvent systems for polyamide-imide and/or polyimide resins are known in the art. U.S. Pat. No. 3,501,443 discloses a process for the preparation of polyimide resins in the presence of acetophenone, cyclohexanone, isophorone, proprophenone or benzophenone. U.S. Pat. No. 3,607,814 shows a composition comprising a polyimide and a mixture of solvents comprising butyrolactone along with cyclohexaneone, acetophenone, isophorone, benzophenone or propisophenone wherein the solids content of the polyimide in solution is between 5 and 30 percent by weight based on the total weight of solution. U.S. Pat. No. 3,677,992 teaches the use of a mixture of a phenol or a substituted phenol or o-, m-, or p-cresol along with aromatic ketones such as benzophenone and acetophenone as a solvent system for polyamide-imide resins as magnet wire coating compositions. U.S. Pat. Nos. 3,884,880 and 3,937,673 disclose the use of acetophenone and N-methyl pyrrolidone as reaction sovents, the reaction being quenched by adding aprotic or polar solvents or phenolic solvents such as cresylic acid, phenol, cresol or the like.